1. Technical Field
The present invention generally relates to methods and systems used to attach structural members, more specifically, to a joint for connecting conduits, and particularly to a joint that may be used to provide both mechanical attachment and a complete pressure seal for polymeric (plastic) pipe.
2. Description of Related Art
Many types of conduits are used to provide various utility services, such as telephone, cable, water, waste water, and natural gas. These conduits come in a variety of sizes, shapes and materials. One of the most common forms is a pipe having a circular cross-section, constructed of a polymeric (plastic) material, particularly acrylonitrile butadiene styrene, or polyvinyl chloride. These specific materials are typically used to construct pipes that are relatively rigid.
Conventional distribution systems for these various utilities use polymeric pipes of varying diameters to transport product, or convey signal lines, from suppliers to end users. There is a continual problem regarding leakage in such pipe systems due to inadequate joining of pipe sections. Leakage may be from the inside of the pipe to the outside (e.g., water or natural gas) resulting in the loss of product, or from the outside of the pipe to the inside (e.g., water penetration) which may deteriorate cabling that is protected by the conduit.
Various methods and systems have been devised to join the ends of plastic pipes. One approach is the use of adhesives (see, e.g., U.S. Pat. Nos. 3,784,235, 3,909,045 and 3,826,521). Joining pipe using adhesives, however, is generally unacceptable due to the difficult adherence characteristics of many polymers, which can result (after a period of time) in cracks in the cured adhesive or in separation of the adhesive from the pipe surface. The use of adhesive or glue (which may be toxic) also often involves a change in the inner diameter or outer diameter of one or both of the pipes, such as with a bell-and-pipe slip joint. Non-uniform pipe diameters can introduce difficulties when installing the pipe.
There is a further unmet need with regard to the joining of dissimilar plastic pipes, such as polyethylene (PE) and polyvinyl chloride (PVC). Present methods depend primarily on mechanical type couplings which are costly, use a variety of sealing methods such as “O” rings to achieve a satisfactory performance based joint. Solvent-based joining materials do not achieve any level of bond strength on polyethylene and other low-surface energy piping materials. Connecting polyethylene pipe to the existing underground network of PVC requires mechanical fittings. In the electrical distribution industry where polyethylene conduit must be joined to the flame retardant PVC sweep riser for service entrance a need exists for a smooth wall (Inside/Out) pipe joining method.
Another technique for fitting pipes together involves the use of electrofusion couplers (see, e.g., U.S. Pat. No. 4,486,650). These couplers overcome the adherence properties of the polyolefin compositions by being formed of the same thermoplastic material as the conduits to be joined. An electrical resistance heating element within the coupler raises the temperature of the material in both the coupler and the adjoining pipe, which then flows together. Upon cooling, the material from the two parts fuses together. As with other prior art techniques, however, electrofusion couplers often do not create a complete seal and the entire process must sometimes be repeated. Electrofusion couplers are also relatively expensive.
A more complicated technique for joining plastic pipe, using microwaves, is disclosed in U.S. Pat. No. 5,254,824. That system requires a composite bonding material comprised of a plurality of coated susceptor particles suspended in a matrix. As with the electrofusion couplers, the material of the matrix is adapted to fuse with the material of the pipe at high temperatures. The material is heated by energization of the susceptor particles via radiant microwave energy. This system is very complicated, however, as it requires a special microwave cavity and power supply, both of which can be very bulky. This technique also presents safety hazard to workers who may be exposed to microwave radiation.
A simpler approach to joining pipes is the use of pipe threading, i.e., where one pipe (with exterior male threads) screws into the end of the second pipe (with interior female threads). While threading is an acceptable way of joining certain kinds of pipe, particularly metal pipe, it is again much less useful in joining plastic pipe. Plastic threads do not have the strength of metal threads, and are more susceptible to stripping. Also, when a straight thread is used to connect two cylinders, a deep shoulder is required to accommodate each thread (male and female), which accordingly decreases the longitudinal strength of the cylinder wall. To avoid this problem, it is necessary to alter either the inner diameter or outer diameter of one of the pipes, which may lead to the difficulties mentioned above.
An alternative molded plastic cylinder thread design is disclosed in NASA Tech Brief 71-10336. That design uses a specific molded Spiral Buttress Thread with a 30 degree taper. With the Spiral Buttress design, no shoulder is required, removing the concerns regarding compromised wall thickness. The Spiral Buttress Thread is generally easy to start (that is, to align the male and female threads) and may, depending upon the initial relative axial orientation of the two pipe ends, be fully engaged in less than one relative rotation of the pipe ends, regardless of their diameter. This design, however, still does not provide a sufficient mechanical connection or a reliable high pressure seal. It would, therefore, be desirable to devise an improved pipe joint which both mechanically affixes two pipe ends and provides a stronger pressure seal. It would be further advantageous if the improved joint could be quickly completed without the need for special tools or equipment.